1. Field of the Invention
The present invention relates to a process for the surface treatment of N-alkyl-N'-phenyl-p-phenylenediamine particles, in which the surface properties of N-alkyl-N'-phenyl-p-phenylenediamine particles are changed by causing the particles to freely flow in a vessel maintained at a temperature lower than the melting point of the diamine.
N-Alkyl-N'-phenyl-p-phenylenediamines, for example, N-isopropyl-N'-phenyl-p-phenylenediamine and N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, are used in large quantities as stabilizers such as antioxidants for rubbers or polymerization inhibitors for unsaturated carboxylic acids or esters thereof, and are marketed in the form of powders, flakes or granules.
2. Description of the Related Art
The above-mentioned marketed products of N-alkyl-N'-phenyl-p-phenylenediamines, especially flakes, are readily broken and powdered during transportation, and as in the case of powders, dust is scattered during use thereof and the working environment is deteriorated. Moreover, in the case of a compound having a low melting point, such as N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, formation of a small amount of powder causes agglomeration and a compaction phenomenon is induced. Therefore, fine powder contained in the product must be removed to avoid the occurrence of this phenomenon, and accordingly, a complicated sieving operation becomes necessary, drastically reducing the yield.
As the result of recent rationalization in the manufacture of rubber products such as tires, automatic metering is performed at the compounding step and this necessitates that rubber additives such as antioxidants be weighed continuously and in a given quantity. It is considered that granulation of the N-alkyl-N'-phenyl-p-phenylenediamines makes a great contribution to the above rationalization.
As means for granulating N-alkyl-N'-phenyl-p-phenylenediamines, there have been proposed, for example, a process in which a melt of an N-alkyl-N'-phenyl-p-phenylenediamine is incorporated or dropped into an aqueous solution of a hydrophilic polymeric compound or a surface active agent and the mixture is solidified, and a process in which the melt is dropped onto a steel belt to effect solidification. Furthermore, a process is known in which a powdery N-alkyl-N'-phenyl-p-phenylenediamine is dispersed in water containing an organic solvent and the dispersion is heated at a temperature lower than the boiling point of water to effect granulation.
In the conventional processes, granular products are obtained by rapidly cooling melts, and therefore, the particles are brittle and fine powders are generated by breakage during transportation or in a hopper; although the resistance to disintegration is improved as compared with flaky products. The pulverized fine powders are accumulated in the bottom portion of the hopper and have poor flowability. Accordingly, agglomeration or compaction occurs; making automatic metering difficult and clogging the discharge opening of the hopper. Especially, when the melt is dropped on the steel belt, the formed particles are obtained in the dry state, and fine powders formed by breakage during transportation in the preparation process adhere to the surfaces of other particles to reduce the flowability of those particles. Furthermore, these fine powders are scattered as dust and it is impossible to completely avoid contamination of the working environment. In the process in which a powdery N-alkyl-N'-phenyl-p-phenylenediamine is dispersed in water, dust scattering is caused at the dispersing step, also contaminating the environment. Moreover, the process of recovering the organic solvent becomes necessary, which is not advantageous from the industrial viewpoint.